Process for removing polymeric binders from nonwoven fabrics

ABSTRACT

Certain copolymers of vinyl acetate and acrylic esters, used to bond fibers to form nonwoven fabrics, can be removed from such fabrics by an alkaline treatment, without degradation of the fiber. The fibers, in separated and unbonded form, may then be reprocessed into webs.

United States Patent 1191 Mitchell June 11, 1974 PROCESS FOR REMOVING POLYMERIC 3,262,838 7/1966 Vieth et al. 162/8 3,630,831 12/1971 Jongetjes 162/168 x BINDERS FROM NONWOVEN FABRICS [75] Inventor: Philip A. Mitchell, East Walpole,

Mass.

[73] Assignee: The Kendall Company, Walpole,

Mass.

[22] Filed: Apr. 30, 1973 [2]] App]. No: 355,959

[52] US. Cl. 162/8 [51] lnt. Cl. D2lc 5/02 [58] Field of Search 162/4, 8, 191, 168

[56] References Cited UNITED STATES PATENTS 14166 et a1. 162/168 x Primary Examiner-Robert L. Lindsay, Jr. Assistant Examiner-William F. Smith Attorney, Agent, or Firm-John F. Ryan [57] ABSTRACT Certain copolymers of vinyl acetate and acrylic esters, used to bond fibers to form nonwoven fabrics, can be removed from such fabrics by an alkaline treatment, without degradation of the fiber. The fibers, in separated and unbonded form, may then be reprocessed into webs.

3 Claims, N0 Drawings PROCESS FOR REMOVING POLYMERIC BINDERS FROM NONWOVEN FABRICS BACKGROUND OF THE INVENTION Nonwoven fabrics, from a modest beginning about 35 years ago, have been an important staple article of commerce, accounting annually for the consumption of hundreds of millions of pounds of fiber. Although a minor proportion of such nonwoven fabrics is made by the use of thermoplastic binder fibers, or by a needling process, or by a felting type of mechanical fibrous interlocking, by far the major portion of commercial nonwoven fabrics is made by applying an aqueous suspension or dispersion of a polymeric bonding agent to an unspun and unwoven array of textile-length fibers, removing excess binder if any, and drying the array to remove the water and to set the fabric in a bonded condition.

In the woven textile industry, looms are available which can weave fabrics exactly to so-called standard widths: 36 inches 48 inches, 60 inches, etc., in width, with no selvage trimming or edge waste. Nonwoven fabrics, however, are made from loose fibrous fleeces from cards, garnetts, or air-lay machines, and are characterized by irregular edges which must be trimmed. Additionally, a large volume of nonwoven fabrics is sold in the form of slit rolls, resulting in additional waste where the number of slittable rolls is not evenly divisible into the width of the mill-run roll.

This combination of circumstances results in a tremendous annual accumulation of trimmings and waste. Unlike the situation in the woven textile or paper industry, where the waste fiber content can be reclaimed by mechanical or hydraulic means, the bonded fibers in nonwoven fabrics cannot be similarly reclaimed and restored to cardable condition due to the firm adhesion of the bonding polymer to the fibers.

SUMMARY OF THE INVENTION It has now been found that a certain restricted class of binders, suitable for bonding nonwoven fabrics for numerous industrial and consumer end uses, can be dissolved and removed from such fabrics by a simple and economical aqueous process, without substantial degradation of the fiber, which after rinsing, neutralizing, and drying is suitable for reuse, in being cardable without breakage.

It is the primary object of this invention to provide such a process.

It is known that strong caustic solutions, such as aqueous solutions of sodium hydroxide, will soften or dissolve the impurities associated with raw cotton, as is commonly practiced in the pressure kier-boiling of cotton. Such a process will also soften or dissolve certain polymeric binders, especially those with a substantial saponifiable ester content. Unfortunately, pressure kier-boiling, or boiling at atmospheric pressure for prolonged periods in the presence of air, has a strongly degradative effect on such fibers as viscose rayon, as well as on certain other synthetic fibers to a lesser degree.

Viscose rayon, subjected to a prolonged alkaline'boil, suffers a loss in its degree of polymerization, and may become so weak and embrittled that it is not strong enough to withstand the mechanical stresses of a carding operation.

I have found, however, that a certain class of binders of the copolymer type, wherein both monomers are esteric in nature, can be substantially removed from the fibers in a bonded nonwoven fabric by a relatively mild alkaline treatment to provide separated cardable fibers. More specifically, the alkaline treatment preferred in this'invention involves heating, preferably by boiling the fabric at atmospheric pressure, in an aqueous sodium hydroxide solution which ranges from about 1 to 5 percent concentration of hydroxide for less than 2 hours, preferably from 1 percent sodium hydroxide for.

2 hours to 5 percent sodium hydroxide for 15 minutes. A convenient and operable liquor to fabric ratio is 20 to l.

It has been found that only a restricted number of polymeric bonding agents are cleanly stripped from nonwoven fabrics by such a treatment. First of all, percent or more of the polymer must be a copolymer of vinyl acetate and an ester of acrylic acid in which the ester portion of the acrylic polymer contains from one to four carbon atoms. As a second requirement, the vinyl acetate moiety of the copolymer must constitute the major portion thereof, based on a mole ratio calculation.

For example, the following polymers have been found to be readily removable from nonwoven fabrics made up of viscose rayon fibers bonded with:

vinyl acetate-methyl acrylate copolymers in a 53 to 47 mole ratio; vinyl acetate-ethyl acrylate copolymers in a 55 to 45 mole ratio; vinyl acetate-butyl acrylate copolymers in a 56 to 44 mole ratio.

The ratio of vinyl acetate to the acrylic ester in the copolymer is apparently quite critical. When a nonwoven fabric bonded with a vinyl acetate (0.48 mole) butyl acrylate (0.52 mole) binder was boiled for 15 minutes in 5 percent sodium hydroxide solution, the binder softened but did not strip from the fibers to a degree which would allow them to be recarded.

A second point of criticality is the use of an acrylic ester. Although a vinyl acetate-butyl acrylate copolymer in a 56 to 44 mole ratio yielded clean, cardable fibers, a vinyl acetate-butyl methacrylate copolymer in the same mole ratio did not strip from the fibers when boiled with dilute sodium hydroxide.

A third critical consideration is the size of the alkyl group in the ester portion of the acrylic ester. Although the methyl, ethyl, and butyl esters stripped readily, as set forth above, acopolymer of vinyl acetate 2- ethylhexyl acrylate in a 56 to 44 mole ratio could not be stripped under the same conditions.

It is possible to crosslink the copolymers found usable in this invention, to a modest degree, without destroying their capacity for being stripped from rayon fibers by an alkaline treatment. The incorporation into the copolymer of a small mole fraction of an agent such as N-methylolacrylamide, usually 0.05 mole percent or less, yields strippable binders. Similarly, emulsion stabilizers of the quaternary ammonium type, as set forth in copending U.S. Pat. application Ser. No. 110,588, of common assignee, may be used to lend stability to the 3, copolymer emulsion, without any adverse effect on strippability.

Additionally, minor mole amounts of modifying monomers may be copolymerized with the vinyl acetate-acrylic ester copolymer, to alter the hand, stiffness, tenacity, etc., of the bonding polymer. For example, a copolymer of vinyl acetate-butyl acrylateglycidyl acrylate-hydro'xyethyl acrylate, in which the mole ratios were 53, 41, 3, and 3, respectively, stripped readily in hot dilute alkaline solution. The criterion for strippability seems to be that the vinylacetate-acrylic ester portions of the polymer together account for at least 90 percent of the polymer, by mole fraction, and that the vinyl acetate fractionper se account for over 50 percent, mole fraction, of the total polymer.

This effect of vinyl acetate upon acrylic esters with which it is copolymerized is quite unexpected, since it does not appear at lower acetate levels. For example, films were cast from copolymers of vinyl acetate-butyl acrylate, and were then boiled in 2 percent sodium hydroxide for 1/2 hour. Films cast from 100 percent butyl acrylate showed less than 3 percent weight loss. In films from 14 mole percent vinyl acetate 86 mole percent butyl acrylate, the loss was 3.7 percent. For 27 acetate 73 acrylate films, the loss'was 4.5 percent, but from 55 acetate 45 acrylate films, the loss was 100 percent, the film dissolving completely. The effect of increasing vinyl acetate content is thus non-linear, and

the behavior of such copolymers at percent or more vinyl acetate content could not be anticipated.

Having thus described my invention, I claim:

I. The process of removing from nonwoven fabrics a polymeric bonding agent comprising a copolymer of vinyl acetate and an ester of acrylic acid, said copolymer constituting at least 90 percent, by mole fraction, of said bonding agent,

the vinyl acetate moiety constituting over 50 mole percent of said copolymer,

and the acrylic ester constituting less than 50 percent mole fraction of said copolymer, and said acrylic ester having between one and four carbon atoms in the ester group,

which comprises heating said bonded nonwoven fabric in an aqueous solution of sodium hydroxide having a concentration between 1.0 to 5.0 percent sodium hydroxide, at atmospheric pressure, for a period between 15 minutes to 2 hours,

whereby separated cardable fibers are produced from said nonwoven fabric.

2. The process according to claim 1 in which the nonwoven fabric comprises viscose rayon fibers.

3. The process according to claim 1 in which the vinyl acetate-acrylic ester copolymer is crosslinked with a small amount of a crosslinking agent. 

2. The process according to claim 1 in which the nonwoven fabric comprises viscose rayon fibers.
 3. The process according to claim 1 in which the vinyl acetate-acrylic ester copolymer is crosslinked with a small amount of a crosslinking agent. 